1. Field of the Invention
This invention relates to an RF modulator for converting an output signal, for example, of a magnetic picture recording device into a signal adapted for inputting into a television set.
2. Description of the Prior Art
For reference, a circuit example of the conventional RF modulator formed by a one-chip IC and an surface acoustic wave resonator is shown in FIG. 4.
In FIG. 4, to resonance element connection terminals 9 and 10 of a one-chip IC 20 an elastic surface-wave resonator 30 of a given property is connected. Other than the resonance element connection terminals 9 and 10, the IC 20 is provided with ground terminals 1 and 14, audio modulation signal output terminal 2, terminal 3 for impedance matching of an audio circuit, terminals 4 and 5 for connection of an audio intercarrier tuning element, audio signal input terminal 6 to which an audio signal is applied via a pre-emphasis circuit, channel select terminal 7, power supply terminals 8 and 11, high-frequency grounding terminals 12 and 13, video modulation output terminal 15, and video signal input terminal 16. In the IC 20 there are incorporated functional blocks, such as clamp circuit 21, white clip circuit 22, video signal modulator 23, carrier oscillator 24, RF amplifier 25, audio amplifier 26, FM modulator 27, and audio signal mixer 28. Further, to respective terminals of the IC 20 peripheral circuits are connected appropriately.
In the RF modulator of the foregoing configuration, the carrier oscillator 24 oscillates in response to the resonance frequency of the surface acoustic wave resonator 30 to output a carrier wave. This carrier wave is amplified by the RF amplifier 25 and applied to the video signal modulator 23 and audio signal mixer 28. The video signal applied to the video signal input terminal 16 is processed appropriately by the clamp circuit 21 and white clip circuit 22 and applied to the video signal modulator 23, then, it amplitude-modulates the carrier wave given from the carrier oscillator 24 and the result is supplied to the video modulation signal output terminal 15. The audio signal applied to the audio signal input terminal 6 is amplified by the audio amplifier 26, converted by the FM modulator 27 into an audio-modulated FM wave, for example, of 4.5 MHz, and applied to the audio signal mixer 28. In the audio signal mixer 28, this FM wave and the carrier wave given from the carrier oscillator 24 are mixed together and output to the audio modulation signal output terminal 2.
In the conventional RF modulator incorporating the surface acoustic wave resonator 30 as shown in FIG. 4, because the frequency precision and stability of the surface acoustic wave resonator 30 are excellent, the frequency and output level of the carrier wave provided from the carrier oscillator 24 are stable. In addition, the gain of the RF amplifier 25 for amplifying the carrier wave is pre-set so that the carrier wave can be applied at the optimum level to the video signal modulator 23 and audio signal mixer 28 depending upon the given property of the surface acoustic wave resonator 30.
In the foregoing configuration, if the surface acoustic wave resonator 30 of the given property serving as the resonator is replaced by a crystal resonator of different structure, or a resonance element composed of coil and condenser, or another one of different frequency, the level and/or frequency of the carrier wave provided from the carrier oscillator 24 becomes different from that of the previous combination, and the level of the carrier wave supplied to the video signal modulator 23 and audio signal mixer 28 may or may not assume the optimum state. That is, if the input level of the carrier wave is too low the sensitivity of modulation decreases, whereas if it is too high a carrier leak arises. Particularly, due to the carrier leak there occurs incidental carrier phase modulation (ICPM). As a result, the signal-to-noise (SN) ratio and the characteristics, such as differential phase (DP) and differential gain (DG), of the RF modulator are degraded, thereby resulting in the problem that the quality of picture and of sound being reproduced, for example, in the television set is poor.